arm64: enable generic idle loop

Enable generic idle loop for ARM64, so can support for hlt/nohlt
command line options to override default idle loop behavior.
Acked-by: NCatalin Marinas <catalin.marinas@arm.com>
Signed-off-by: NLeo Yan <leo.yan@linaro.org>
Signed-off-by: NWill Deacon <will.deacon@arm.com>

arch/arm64/Kconfig

@@ -32,6 +32,7 @@ config ARM64
 	select GENERIC_CLOCKEVENTS_BROADCAST
 	select GENERIC_CPU_AUTOPROBE
 	select GENERIC_EARLY_IOREMAP
+	select GENERIC_IDLE_POLL_SETUP
 	select GENERIC_IRQ_PROBE
 	select GENERIC_IRQ_SHOW
 	select GENERIC_IRQ_SHOW_LEVEL